What is pin goal strength for Hair Dynamics?
The Blender Manual describes it as:

Spring stiffness of the vertex target position.

To my understanding, this is how much the hair wants to "spring" back into its initial/styled position. A lower value makes the hair looser and a higher value makes it stiffer.

This is something probably best understood by simply experimenting with the value and seeing how it affects the simulation (this description just comes from my own experience).